1. Field of the Invention
The invention relates to a method of manufacturing composite sheet steel of maraging steel, especially for the protection of vehicles, in particular, passenger cars and transport vehicles for valuables, against shots and the effect of explosives, wherein the composite sheet steel comprises a harder outer layer and a more tenacious inner layer.
2. Description of the Related Art
Such a composite sheet steel is known from DE 43 44 879 C2.
The outer layer contains 4.0 to 6.0% Mo, 17.0 to 18.0% Ni,  less than 0.05% Cr, 1.7 to 1.8% Ti, and 14.0 to 15.0% Co, while the inner layer contains 4.0 to 6.0% Mo, 17.0 to 18.0% Ni,  less than 0.08% Cr, 0.5 to 0.8% Ti, and 7.0 to 9.0% Co. Both layers contain moreover  less than 0.02% C,  less than 0.06% Si,  less than 0.01% Mn,  less than 0.01% P,  less than 0.01% S, and under  less than 0.02% Cu, with the remainder being Fe and contaminants resulting from the manufacturing process.
A contact surface is to be made planar and to be cleaned by planing on two blocks of the two steels, respectively, whereupon the blocks are placed onto one another and pressed against one another and connected to one another by a peripherally extending welding seam. After being heated to approximately 1300xc2x0 C., they are to be rolled out with a roll separating force of approximately 8xc3x97104 kN.
Based on the data of this patent, it is not easily possible to produce composite sheet steels which fulfill the requirements.
It is an object of the present invention to provide a method for manufacturing a sheet steel providing a high safety against shots and the effect of explosives.
In accordance with the present invention, this is achieved in connection with a method of the aforementioned kind in that the steel of the inner layer is produced with a chemical composition in percent by weight of Cxe2x89xa60.01, Sixe2x89xa60.1, Mnxe2x89xa60.1, Pxe2x89xa60.005, Sxe2x89xa60.005, Cuxe2x89xa60.1, Mo 4.80 to 5.20, Ni 17.5 to 18.5 , Crxe2x89xa60.1, Ti 0.55 to 0.70, Co 8.0 to 9.0 as well as optionally Al 0.05 to 0.15, and in that the steel of the outer layer is produced with a chemical composition, obtained by purification by means of zone melting, in percent by weight of Cxe2x89xa60.01, Si less than 0.1, Mn 0.02 to 0.20, Pxe2x89xa60.005, Sxe2x89xa60.005, Cu 0.01 to 0.20, Mo 4.80 to 5.20, Ni 17.5 to 18.5, Cr 0.01 to 0.20, Ti 1.80 to 1.95, Co 14.0 to 15.5, Al 0.05 to 0.15, with the remainder being Fe and contaminants resulting from the manufacturing process, respectively.
A composite sheet steel produced accordingly fulfills high requirements which will be explained in more detail in the following.
In comparison, to the manufacture of the above-mentioned known composite sheet steel, according to the invention at least the outer layer has a different construction.
In regard to the chemical composition the outer layer differs, in particular, by the added component aluminum, by a different and higher contents of Ti and Mn, and by a substantially different and higher contents of Cu. Moreover, the maximum contents of C, P, and S is reduced, respectively, and the Mo range is greatly limited. Of great importance in this connection is the added method step of zone melting.
The aluminum contents of 0.05 to 0.15% still present after the zone melting step ensures a high oxide purity degree. The aluminum contents, surpassing the above mentioned contents prior to the zone melting process, prevents the formation of other oxides, especially of TiO and Ti2O3, and separates together with the oxygen bonded thereto from the slab during the zone melting process. The steel is thus substantially free of oxide inclusions which, according to the assumption of the invention, should be avoided as much as possible because of their notch effect causing cracks, in particular, during loading of the steel when subjected to shots, and this cannot be neglected in this context.
The reduction of the segregation tendency as a result of the decisive reduction of the contents of phosphorus and sulfur is to be considered approximately along the same lines.
The suggested increased contents of Mn effects an increased solid solution formation in the nickel martensite which increases the strength and hardness.
In the same way, the strength and hardness are further improved by the increase of the copper contents.
In the steel of the inner layer, however, Mn is limited to a maximum of 0.1%, and Cu is reduced to at most half the value in order not to impair the desired tenacity in the inner layer.
In the same sense, but with greater overlap with respect to the analysis of the known composite steel, a minimum contents of Cr is provided for the outer layer as well as a substantially increased maximum contents. In contrast thereto, the Cr contents remains limited for the inner layer.
The stringent limitation of the Mo contents is desirable in order to achieve a reproducible optimum for the overall effect of the provided alloy elements and alloy contents.
Since according to the invention not only the Ti contents is increased but moreover the C contents is limited to half the value, less Ti is bound by C and, accordingly, the strength-increasing and hardness-increasing effect of titanium in the steel of the outer layer is taken advantage of to an even greater extent, especially by inclusion in the intermetallic phases. At the same time, an improved tenacity behavior is achieved during the greatly dynamic loading when subjected to shots.
On the other hand, the Ti contents is reduced in the steel of the inner layer in order to adjust a favorable tenacity.
For the steel of the inner layer a zone melting is not an absolute requirement, but is nonetheless advantageous. The same holds true for the component aluminum which is optional in the inner layer.
Because of the desired purity, both steels should be melted under vacuum, preferably by vacuum induction melting.
The mechanical manufacture of the composite sheet steel is carried out preferably such that the outer layer and the inner layer, after cleaning, for example, by grinding, of the contact surfaces, are connected by explosive cladding and subsequent rolling of two plates comprised of the two steels.
However, possible is also a pure roll-bonding process. However, in this connection, metallic contact surfaces on the two plates to be connected should be provided by a metal removing machining process such as planing or milling.
Moreover, before the roll-bonding process is carried out, the plates should be welded together tightly peripherally and a high vacuum should be generated between the two plates.
The best resistance of the composite sheet steel against shots can be achieved when for the safety requirements according to EN (Euronorm) 1522, shot class (xe2x80x9cBeschuxcex2klassexe2x80x9d) B7 (hardcore ammunition), the thickness ratio between the outer layer and the inner layer is selected to be between 1.5:1 and 4:1 and for the safety requirements according to EN 1522, shot classes (xe2x80x9cBeschuxcex2klassexe2x80x9d) B6/B5, is selected to be between 0.3:1 and 1:1.